Fuel cell interconnect with reduced voltage degradation and manufacturing method

ABSTRACT

A method of making an interconnect for a solid oxide fuel cell stack includes contacting an interconnect powder located in a die cavity with iron, the interconnect powder including a chromium and iron, compressing the interconnect powder to form an interconnect having ribs and fuel channels on a first side of the interconnect, such that the iron is disposed on tips of the ribs; and sintering the interconnect, such that the iron forms an contact layer on the tips of the ribs having a higher iron concentration than a remainder of the interconnect. A glass containing cathode contact layer having a glass transition temperature of 900° C. or less may be located over the rib tips on the oxidant side of the interconnect.

FIELD

The present invention is directed to fuel cell stack components,specifically to interconnects and methods of making interconnects forfuel cell stacks.

BACKGROUND

A typical solid oxide fuel cell stack includes multiple fuel cellsseparated by metallic interconnects (IC) which provide both electricalconnection between adjacent cells in the stack and channels for deliveryand removal of fuel and oxidant. The metallic interconnects are commonlycomposed of a Cr based alloy such as an alloy known as CrFe which has acomposition of 95 wt % Cr-5wt % Fe, or Cr—Fe—Y having a 94 wt % Cr-5 wt% Fe-1 wt % Y composition. The CrFe and CrFeY alloys retain theirstrength and are dimensionally stable at typical solid oxide fuel cell(SOFC) operating conditions, e.g. 700-900 C in both air and wet fuelatmospheres. However, during operation of the SOFCs, chromium in theCrFe or CrFeY alloys react with oxygen and form chromia, resulting indegradation of the SOFC stack.

Two of the major degradation mechanisms affecting SOFC stacks aredirectly linked to chromia formation of the metallic interconnectcomponent: i) higher stack ohmic resistance due to the formation ofnative chromium oxide (chromia, Cr₂O₃) on the interconnect, and ii)chromium poisoning of the SOFC cathode.

Although Cr₂O₃ is an electronic conductor, the conductivity of thismaterial at SOFC operating temperatures (700-900 C) is very low, withvalues on the order of 0.01 S/cm at 850 C (versus 7.9×10⁻¹ Scm for Crmetal). The chromium oxide layer grows in thickness on the surfaces ofthe interconnect with time and thus the ohmic resistance of theinterconnect and therefore of the SOFC stack due to this oxide layerincreases with time.

The second degradation mechanism related to the chromia forming metallicinterconnects is known as chromium poisoning of the cathode. At SOFCoperating temperatures, chromium vapor diffuses through cracks or poresin the coating and chromium ions can diffuse through the lattice of theinterconnect coating material into the SOFC cathode via solid statediffusion. Additionally, during fuel cell operation, ambient air (humidair) flows over the air (cathode) side of the interconnect and wet fuelflows over the fuel (anode) side of the interconnect. At SOFC operatingtemperatures and in the presence of humid air (cathode side), chromiumon the surface of the Cr₂O₃ layer on the interconnect reacts with waterand evaporates in the form of the gaseous species chromium oxidehydroxide, CrO₂(OH)₂. The chromium oxide hydroxide species transports invapor form from the interconnect surface to the cathode electrode of thefuel cell where it may deposit in the solid form, Cr₂O₃. The Cr₂O₃deposits on and in (e.g., via grain boundary diffusion) the SOFCcathodes and/or reacts with the cathode (e.g. to form a Cr—Mn spinel),resulting in significant performance degradation of the cathodeelectrode. Typical SOFC cathode materials, such as perovskite materials,(e.g., LSM, LSC, LSCF, and LSF) are particularly vulnerable to chromiumoxide degradation.

SUMMARY

According to various embodiments, provided is a method of forming aninterconnect for a solid oxide fuel cell stack, the method comprising:contacting an interconnect powder located in a die cavity with iron, theinterconnect powder comprising chromium and iron; compressing theinterconnect powder to form an interconnect comprising ribs and fuelchannels on a first side of the interconnect, such that the iron isdisposed on tips of the ribs; and sintering the interconnect, such thatthe iron forms an contact layer on the tips of the ribs having a higheriron concentration than a remainder of the interconnect.

According to various embodiments, provided is a method of forming aninterconnect for a solid oxide fuel cell stack, the method comprising:filling a die cavity with an interconnect powder comprising a chromiumalloy; compressing the interconnect powder to form an interconnectcomprising ribs configured to form fuel channels on a first side of theinterconnect; disposing iron on tips of the ribs; and sintering theinterconnect, such that the iron forms a contact layer on the tips ofthe ribs.

According to various embodiments, provided is a fuel cell interconnect,comprising oxidant channels at least partially defined by first ribsdisposed on a first side of the interconnect, fuel channels at leastpartially defined by second ribs disposed on an opposing second side ofthe interconnect, and a cathode contact layer located only over tips ofthe second ribs, the cathode contact layer comprising a conductive metaloxide and a glass material having a glass transition temperature of 900°C. or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a SOFC stack, according to variousembodiments of the present disclosure.

FIG. 1B is a cross-sectional view of a portion of the stack of FIG. 1A.

FIG. 2A is a top view of an air side of an interconnect, according tovarious embodiments of the present disclosure.

FIG. 2B is a top view of a fuel side of the interconnect of FIG. 2A.

FIG. 3 is a sectional view of a fuel-side rib of an interconnect and ananode of an adjacent fuel cell, according to various embodiments of thepresent disclosure.

FIG. 4 is a sectional view of a fuel cell connected to the air side ofan interconnect, according to various embodiments of the presentdisclosure.

FIGS. 5A-5E illustrate a method of forming an interconnect, according tovarious embodiments of the present disclosure.

FIGS. 6A-6E illustrate a method of forming an interconnect, according tovarious embodiments of the present disclosure.

FIG. 7A illustrates a method of forming contact regions on aninterconnect, according to various embodiments of the presentdisclosure.

FIG. 7B illustrates an alternative to the method of FIG. 7A, accordingto various embodiments of the present disclosure.

FIG. 8 is a chart showing area specific resistance degradation (ASRD)values of exemplary and comparative interconnects disposed in a fuelcell stack, according to various embodiments of the present disclosure.

FIG. 9 is a chart showing area specific resistance degradation (ASRD)values of exemplary and comparative interconnects, according to variousembodiments of the present disclosure.

FIG. 10 is a chart showing area specific resistance degradation (ASRD)values of exemplary and comparative interconnects disposed in a fuelcell stack, according to various embodiments of the present disclosure.

DETAILED DESCRIPTION

FIG. 1A is a perspective view of a solid oxide fuel cell (SOFC) stack100, and FIG. 1B is a sectional view of a portion of the stack 100,according to various embodiments of the present disclosure. Referring toFIGS. 1A and 1B, the stack 100 includes fuel cells 1 separated byinterconnects 10. Referring to FIG. 1B, each fuel cell 1 comprises acathode electrode 3, a solid oxide electrolyte 5, and an anode electrode7.

Various materials may be used for the cathode electrode 3, electrolyte5, and anode electrode 7. For example, the anode electrode 3 maycomprise a cermet comprising a nickel containing phase and a ceramicphase. The nickel containing phase may consist entirely of nickel in areduced state. This phase may form nickel oxide when it is in anoxidized state. Thus, the anode electrode 7 is preferably annealed in areducing atmosphere prior to operation to reduce the nickel oxide tonickel. The nickel containing phase may include other metals inadditional to nickel and/or nickel alloys. The ceramic phase maycomprise a stabilized zirconia, such as yttria and/or scandia stabilizedzirconia and/or a doped ceria, such as gadolinia, yttria and/or samariadoped ceria.

The electrolyte may comprise a stabilized zirconia, such as scandiastabilized zirconia (SSZ) or yttria stabilized zirconia (YSZ).Alternatively, the electrolyte may comprise another ionically conductivematerial, such as a doped ceria.

The cathode electrode 3 may comprise an electrically conductivematerial, such as an electrically conductive perovskite material, suchas lanthanum strontium manganite (LSM). Other conductive perovskites,such as LSCo, etc., or metals, such as Pt, may also be used. The cathodeelectrode 3 may also contain a ceramic phase similar to the anodeelectrode 7. The electrodes and the electrolyte may each comprise one ormore sublayers of one or more of the above described materials.

Fuel cell stacks are frequently built from a multiplicity of SOFC's 1 inthe form of planar elements, tubes, or other geometries. Although thefuel cell stack in FIG. 1 is vertically oriented, fuel cell stacks maybe oriented horizontally or in any other direction. Fuel and air may beprovided to the electrochemically active surface, which can be large.For example, fuel may be provided through fuel conduits 22 (e.g., fuelriser openings) formed in each interconnect 10.

Each interconnect 10 electrically connects adjacent fuel cells 1 in thestack 100. In particular, an interconnect 10 may electrically connectthe anode electrode 7 of one fuel cell 1 to the cathode electrode 3 ofan adjacent fuel cell 1. FIG. 1B shows that the lower fuel cell 1 islocated between two interconnects 10. As described below, a Ni mesh maybe used to electrically connect the interconnect 10 to the anodeelectrode 7 of an adjacent fuel cell 1.

Each interconnect 10 includes fuel-side ribs 12A that at least partiallydefine fuel channels 8A and air-side ribs 12B that at least partiallydefine oxidant (e.g., air) channels 8B. The interconnect 10 may operateas a gas-fuel separator that separates a fuel, such as a hydrocarbonfuel, flowing to the fuel electrode (i.e. anode 7) of one cell in thestack from oxidant, such as air, flowing to the air electrode (i.e.cathode 3) of an adjacent cell in the stack. At either end of the stack100, there may be an air end plate or fuel end plate (not shown) forproviding air or fuel, respectively, to the end electrode.

Each interconnect 10 may be made of or may contain electricallyconductive material, such as a metal alloy (e.g., chromium-iron alloy)which has a similar coefficient of thermal expansion to that of thesolid oxide electrolyte in the cells (e.g., a difference of 0-10%). Forexample, the interconnects 10 may comprise a metal (e.g., achromium-iron alloy, such as 4-6 weight percent iron, optionally 1 orless weight percent yttrium and balance chromium alloy), and mayelectrically connect the anode or fuel-side of one fuel cell 1 to thecathode or air-side of an adjacent fuel cell 1. An electricallyconductive contact layer, such as a nickel contact layer, may beprovided between anode electrodes 7 and each interconnect 10. Anotheroptional electrically conductive contact layer may be provided betweenthe cathode electrodes 3 and each interconnect 10.

FIG. 2A is a top view of the air side of the interconnect 10, and FIG.2B is a top view of a fuel side of the interconnect 10, according tovarious embodiments of the present disclosure. Referring to FIGS. 1B and2A, the air side includes the air channels 8B. Air flows through the airchannels 8B to a cathode electrode 3 of an adjacent fuel cell 1. Ringseals 20 may surround fuel holes 22A of the interconnect 10, to preventfuel from contacting the cathode electrode. Peripheral strip-shapedseals 24 are located on peripheral portions of the air side of theinterconnect 10. The seals 20, 24 may be formed of a glass orglass-ceramic material. The peripheral portions may be an elevatedplateau which does not include ribs or channels. The surface of theperipheral regions may be coplanar with tops of the ribs 12B.

Referring to FIGS. 1B and 2B, the fuel side of the interconnect 10 mayinclude the fuel channels 8A and fuel manifolds 28. Fuel flows from oneof the fuel holes 22A (e.g., inlet hole that forms part of the fuelinlet riser), into the adjacent manifold 28, through the fuel channels8A, and to an anode 7 of an adjacent fuel cell 1. Excess fuel may flowinto the other fuel manifold 28 and then into the outlet fuel hole 22B.A frame-shaped seal 26 is disposed on a peripheral region of the fuelside of the interconnect 10. The peripheral region may be an elevatedplateau which does not include ribs or channels. The surface of theperipheral region may be coplanar with tops of the ribs 12.

The interconnect 10 may be formed of an alloy containing a high amountof chromium (Cr) which forms a protective chromia shell at hightemperatures. Unfortunately, at these high temperatures, and especiallyin wet air environments, the chromia layer evaporates Cr, most notablyin the form of CrO₂(OH)₂. To help prevent this, the air side surface ofthe interconnect 10 may be coated with an oxide layer to suppress Crevaporation from the interconnect 10 and to reduce the growth of oxidescale.

FIG. 3 is a sectional view illustrating one of the fuel-side ribs 12A ofthe interconnect 10 and the anode 7 of an adjacent fuel cell 1,according to various embodiments of the present disclosure. Referring toFIG. 3, the interconnect 10 is electrically connected to the anode 7 bya Ni mesh 30 covering the fuel side of the interconnect 10. The Ni mesh30 may operate as a current collector with respect to the anode 7.Although only one fuel-side rib 12A is shown, each fuel-side rib 12A ofthe interconnect 10 may be connected to the anode 7 in a similar manner

Conventionally, oxides or other contaminants may form over time on thesurface of an interconnect, and in particular, at an interface between aNi mesh and corresponding ribs of the interconnect. As such, the contactresistance between the Ni mesh and an interconnect may increase, whichmay reduce the useful life of a fuel cell stack.

Referring again to FIG. 3, the interconnect 10 may include a contactlayer 14 configured to prevent or reduce such an increase in contactresistance. In particular, the contact layer 14 may be formed on thetips of the ribs 12A. For example, the contact layer 14 may be disposedat an interface between the interconnect 10 and the Ni mesh 30 (e.g.,between the rib 12A and the Ni mesh 30). In some embodiments, thecontact layer 14 may cover substantially all of the upper (e.g., distal)surface of the rib 12A. However, in other embodiments, the contact layer14 may cover only a portion of the tip of the rib 12A.

The contact layer 14 may have a higher iron content than theinterconnect 10. For example, the contact layer 14 may have greater than7 wt % iron, such as from about 10 to about 95 wt % iron, or about 20 toabout 80 wt % iron, while the interconnect 10 may include an alloyhaving from about 4 to about 6 wt % iron. Accordingly, the contact layer14 may operate to prevent oxide growth at the interface between the rib12A and the Ni mesh 30. The contact layer 14 may also improve themetallurgical joining of the Ni mesh 30 and the interconnect 10.

In some embodiments, the thickness (e.g., depth) and/width of thecontact layer 14 may be controlled to reduce mechanical distortion ofthe interconnect 10. For example, the thickness of the contact layer 14may range from about 5 to about 1000 μm.

The contact layer 14 may have an iron to chromium ratio that varies inthe thickness direction thereof. For example, the iron to chromium ratiomay decrease as a distance from the tip of the rib 12A increases.

According to various embodiments, the contact layer 14 may be formed bydepositing an iron-based material, such as metallic iron or iron oxide,on the tips of the ribs 12A. The interconnect 10 can then be sintered,such that the iron and chromium in the interconnect at least partiallyinter-diffuse, thereby creating the contact layer 14. In particular,iron of the contact layer 14 may partially diffuse into the tip of therib 12A, and chromium of the interconnect 10 may partially diffuse intothe contact layer 14.

In some embodiments, the contact layer 14 may be formed by disposingiron wire on the tips of the ribs 12A. The interconnect 10 may then besintered to facilitate iron and chromium inter-diffusion, as describedabove. The iron wire may have a thickness (e.g., diameter) ranging fromabout 10 to 300 microns, such as from about 15 to about 250 microns, orfrom about 20 to about 200 microns.

In some embodiments, the iron wire may be deposited on an interconnectpowder, and the resultant structure may be compressed into the shape ofan interconnect, followed by sintering. In other embodiments, the ironwire may be deposited on a compressed interconnect, and the resultantstructure may then be sintered.

In some embodiments, the contact layer 14 may be formed by contacting aniron powder to the tip of the rib 12A. The iron powder may have anaverage particle size ranging from about 20 to about 400 microns, suchas from about 25 to about 350 microns, or from about 30 to about 300microns. In some embodiments, the iron powder may be deposited on aninterconnect powder, and the resultant structure may be compressed intothe shape of an interconnect, followed by sintering. For example, theinterconnect powder may be deposited into a die cavity using a firstshoe, and then the iron powder may then be deposited onto theinterconnect powder using a second shoe or by a spraying process.

In other embodiments, the iron powder may be deposited on a compressedinterconnect, or the compressed interconnect may be placed onto the ironpowder ribs down, and the resultant structure may then be sintered.Methods of forming interconnects will be discussed in more detail below.

FIG. 4 is a sectional view of a fuel cell 1 connected to the air side ofan interconnect 10, according to various embodiments of the presentdisclosure. Referring to FIG. 4, a protective coating or layer 40 may bedisposed on the air side of the interconnect 10, and a cathode contactlayer (CCL) 42 may be disposed on the coating 40. In some embodiments,the protective coating 40 and/or the CCL 42 may be deposited only on thetops of the ribs 12B, for example, by a printing method or the like,without depositing the CCL 42 in the oxidant (e.g., air) channels 8B(i.e., without depositing the CCL on or over the sidewalls of the ribs12B and/or without depositing the CCL on or over the bottom of theoxidant channels 8B).

The coating 40 may be configured to limit the diffusion of chromium ions(e.g., Cr³⁺) from the interconnect 10 and into cathode 3 and into seals20, 24. The coating 40 may also be configured to suppress the formationof the native oxide on the surface of the interconnect 10. The nativeoxide is formed when oxygen reacts with chromium in the interconnectalloy to form a relatively high resistance layer of Cr₂O₃. If theinterconnect coating 40 can suppress the transport of oxygen and watervapor from the air to the surface of the interconnect 10, then thekinetics of oxide growth can be reduced.

According to various embodiments, the coating 40 may include a metaloxide spinel material, such as a manganese cobalt oxide (MCO) spinelmaterial and/or a perovskite material, such as lanthanum strontiummanganite (LSM). In an embodiment, the MCO spinel material encompassesthe compositional range from Mn₂CoO₄ to Co₂MnO₄. That is, any spinelmaterial having the composition Mn_(2−x)Co_(1+x)O₄(0≤x≤1) or written asz(Mn₃O₄)+(1−z)(Co₃O₄), where (1/3<z<2/3) or written as (Mn, Co)₃O₄ maybe used, such as Mn₁₅Co₁₅O₄, MnCo₂O₄ or Mn₂CoO₄. The coating 40 may alsobe a mixed layer of MCO and LSM. Many of the spinels that containtransition metals exhibit good electronic conductivities and reasonablylow anion and cation diffusivities and are therefore suitable coatingmaterials. Examples of such materials may be found in U.S. PublishedPatent Application No. 2013/0230792 and U.S. Pat. No. 9,452,475, whichare incorporated herein by reference in their entirety.

The CCL 42 may be an electrically conductive metal oxide layerconfigured to improve an electrical connection between the interconnect10 and the cathode 3. In some embodiments, the CCL 42 may include metaloxide materials that have a low cation diffusivity in the perovskitefamily, such as a lanthanum strontium oxide, e.g., La_(1−x)Sr_(x)MnO₃(LSM), where 0.1≤x≤0.3, such as 0.1≤x≤0.2. In the case of LSM, thematerial has high electronic conductivity yet low anion and cationdiffusion. Other perovskites such as La_(1−x)Sr_(x)FeO_(3−d),La_(1−x)Sr_(x)CoO_(3−d), and La_(1−x)Sr_(x)Co_(1−y)Fe_(y)O_(3−d) allexhibit high electronic conduction and low cation conduction (lowchromium diffusion rates) and may be used as the CCL 42.

Such materials generally have sintering temperatures of more than 1000°C. However, such temperatures may result in the oxidation of metalalloys included in the interconnect 10. As such, it may be difficult toproperly sinter a CCL.

In view of the above and/or other problems, the CCL 42 may include asintering aid configured to increase the density of the CCL 42 and toimprove interfacial strength, and increase layer bonding. In someembodiments, the CCL 42 may include a glass material as a sintering aid.The glass material may be included at an amount less than about 15 wt %,such as from about 1 to about 10 wt %, such as from about 4 to about 10wt %, with the remainder of the CCL 42 being a conductive perovskitemetal oxide, such as LSM. In particular, glass amounts of greater thatabout 15 wt % may unnecessarily reduce the conductivity of the CCL 42.

In addition to better sintering, the addition of the glass material maymake the CCL 42 more compliant and tolerant to mechanical stressesinduced by thermal cycles during SOFC operation. As such, the glassmaterial may be selected from glass materials that have a relatively lowglass transition (Tg) or softening temperature, to avoid crystalization.In particular, the glass material may have a Tg or softening temperaturethat is low enough to allow the glass material to remain viscous attemperatures below 1000° C., such as temperatures of from about 400 toabout 900° C. In some embodiments, selected glass materials may have aglass transition temperature of 900° C. or less, such as 825° C. orless, such as ranging from about 450 to about 550° C. For example, theglass material may remain viscous at fuel cell operating temperaturesranging from about 800 to about 900° C., such as from about 830 to about860° C. Remaining viscous at such temperatures allows the CCL 42 toconform to the tops of the air-side ribs of the interconnect 40.Accordingly, the CCL 42 allows for improved coverage of rib tops, anincreased effective contact surface area, reduced degradation, and theability to self-heal cracks formed during thermal cycling.

According to various embodiments, the glass material may be selectedfrom various compositions, such as alumino-silicate, boro-silicate,boro-aluminate, and alkali-free compositions, and may include Al, Si,Ca, Ba, B, La, Sr, Mg, or mixtures thereof.

FIGS. 5A-5C illustrate a method for forming an interconnect, accordingto various embodiments of the present disclosure. Referring to FIG. 5A,in a first step of the method, an interconnect powder 202 may be addedto a die cavity 201 in a lower punch (die) 200, using a first shoe 206.However, the interconnect powder 202 may be loaded into the die cavity201 by any suitable method. The interconnect powder 202 may include anysuitable interconnect material. For example, the interconnect powder 202may include iron and chromium powders, or may include a chromium alloypowder (e.g., a Cr/Fe alloy powder including 4-6 wt % Fe and a remainderof Cr). The interconnect powder 202 may optionally include a lubricantto facilitate compaction.

Referring to FIG. 5B, in a second step of the method, a contact layermaterial 210 may be disposed on the interconnect powder 202. The contactlayer material 210 may be an iron-containing material, such as metalliciron or iron oxide, and may be in the form of a powder or a wire. Assuch, the contact layer material 210 is referred to herein as iron 210.

As shown in FIG. 5B, iron 210 may be disposed between ribs of a punchpress (i.e., upper die) 208. For example, iron wire or powder may bedisposed between ribs of the punch 208. The iron powder and/or wire mayhave a thickness of from about 20 to about 200 μm. For example, the iron210 may be electro-statically attracted to the punch 208. The punch 208may then compresses the interconnect powder 202 to form an interconnect.

In the alternative, as shown in FIG. 5C, the iron 210 may be disposed onthe interconnect powder 202. The iron 210 may be arranged, such thatwhen the interconnect powder 202 is compressed by the punch 208, theiron 210 is disposed between ribs of the punch 208. When in powder form,the iron 210 may be deposited using a second shoe, a spray depositionmethod, a screen printing method, or the like. When in the form of wire,the iron 210 may be arranged using a jig or the like.

FIG. 5D is a partial sectional view of an interconnect 10 formed by thecompression steps of FIGS. 5B or 5C. As shown in FIG. 5D, thecompression and alignment of the iron 210 results in the formation ofcontact layer 14 on the tips of ribs 12A of the interconnect 10.

The method may then include sintering of the interconnect 10. Thesintering may result in partial inter-diffusion of the interconnectmaterial and the iron of the contact layer 14. As shown in FIG. 5E, acoating material may be applied to the air side of the interconnect 10,prior to the sintering, to form a protective layer 40. The protectivelayer 40 and the contact layer 14 may be sintered at the same time. Inthe alternative, the protective layer 40 may be applied after sinteringthe interconnect 10. The coating material may include, for example, LSMand/or MCO, or a mixture of the coating material and the interconnectpowder 202.

For example, the coating material be applied using a spray method, suchas an air plasma spray (APS), or may be applied using a coating method,such as a wet coating method using a coating material ink. The APSprocess is a thermal spray process in which powdered coating materialsare fed into the coating apparatus. The coating particles are introducedinto a plasma jet in which they are melted and then accelerated towardthe substrate. On reaching the substrate, the molten droplets flattenand cool, forming the coating. The plasma may be generated by eitherdirect current (DC plasma) or by induction (RF plasma). Further, unlikecontrolled atmosphere plasma spraying (CAPS) which requires an inert gasor vacuum, air plasma spraying is performed in ambient air.

The sintering process may be adjusted to keep one or both of the powdersoxidized and/or stable. For example, sintering may be performed usingwet hydrogen, or in an inert atmosphere, such as nitrogen, argon oranother noble gas. The wet hydrogen or inert gas atmosphere is oxidizingor neutral, respectively, and thereby prevents the oxide powder fromreducing.

Although not shown, a CCL may be formed on the protective layer 40. Inaddition, the first step of the method may include depositing a coatingmaterial (e.g., coating powder) in the die cavity 201, prior todepositing the interconnect powder 202. In this manner, the protectivecoating material may form a protective coating primarily on the air sideof the interconnect 10.

According to other embodiments, the method may include applying acoating material after the interconnect powder 202 has been compressed.

The ratio of the coating powder and Fe in the Cr—Fe alloy is preferablyselected so that the coating material has a similar coefficient ofthermal expansion (CTE) to that of the sintered and oxidizedinterconnect. The coefficient of thermal expansion of the Cr—Fe alloy isa function of the composition of the alloy and can be chosen byselecting a Cr to Fe ratio.

In other embodiments, the method may further include applying a cathodecontact layer (CCL) to the protective coating, and then sintering theresultant interconnect. The CCL may be formed from the materialsdescribed above, such as a mixture of a glass material and a conductiveperovskite, such as LSM.

FIGS. 6A-6D illustrate a method of forming an interconnect, according tovarious embodiments of the present disclosure. Referring to FIG. 6A, afirst step of the method includes filling a die cavity 201 with acontact powder 212. In particular, the contact powder 212 may be aniron-based powder, such as metallic iron or iron oxide, for forming acontact layer, as described above (e.g., a 30-300 μm average particlesize powder). An amount of the contact powder 212 may be controlled,such that ribs 203 in the bottom of the die cavity 201 protrude throughthe contact powder 212. In some embodiments, the die cavity 201 may bevibrated to insure the contact powder 212 is disposed between the ribs203 at the bottom of the lower punch 200.

Referring to FIG. 6B, an interconnect powder 202 may be deposited in thedie cavity 201 over the contact powder 212. The interconnect powder 202may be deposited using a shoe or other suitable method, and may includeany of the interconnect materials described herein.

Referring to FIG. 6C, a protective powder 214 may optionally bedeposited on the interconnect powder 202. The protective powder 214 maybe deposited using a shoe or other suitable method, and may include anyof the protective layer materials described herein (e.g., MCO and/orLSM).

Referring to FIG. 6D, a punch press 208 may be used to compress thepowders in the die cavity 201 into the shape of an interconnect 10including contact layer 14 and a protective layer 40, as shown in FIG.6E.

FIG. 7A illustrates a method of forming contact regions on aninterconnect, according to various embodiments of the presentdisclosure. Referring to FIG. 7A, contact powder 212 may be disposed onribs 12A extending from an upper surface of a previously pressed and/orsintered interconnect 10. The interconnect 10 may then be sintered, suchthat the contact powder 212 forms a contact layer 14 at the tips of theribs 12A.

FIG. 7B illustrates an alternative to the method of FIG. 7A, accordingto various embodiments of the present disclosure. Referring to FIG. 7B,a layer of contact powder 212 may be disposed on a substrate 15. Apreviously pressed and/or sintered interconnect 10 may be disposed onthe layer of contact powder 212, such that ribs 12A extending from alower surface of the interconnect 10 contact the contact powder 212. Theinterconnect 10 may then be sintered, such that the contact powder 212forms a contact layer 14 at the tips of the ribs 12A.

FIG. 8 is a graph showing the area specific resistance degradation(ASRD) of interconnects in a solid oxide fuel cell stack, formed frompre-alloyed Cr/Fe powders. The square points are for comparativepre-alloyed interconnects that did not include additional depositediron, and the circular points are for exemplary pre-alloyedinterconnects that included additional deposited iron. As can be seen inFIG. 8, the exemplary interconnects each had a lower average ASRD valuethan that of the comparative interconnects. Further, it can be seen thatthere was a lower contact resistance between the exemplary interconnectsincluding contact layers and a nickel mesh, over time.

FIG. 9 is a graph showing area specific resistances of interconnect tocell interfaces for exemplary CCLs that included glass and comparativeCCLs that did not include glass, over 2000 hours of SOFC stack operatingconditions. Referring to FIG. 9, the exemplary CCLs exhibited lower areaspecific resistances over time, as compared to the comparative CCLs,even though they initially exhibited a higher area specific resistancesover the first 1000 to 2000 hours.

FIG. 10 is a chart showing area specific resistance degradation (ASRD)values of exemplary and comparative interconnects disposed in a solidoxide fuel cell stack, according to various embodiments of the presentdisclosure. The square points are for comparative interconnects that didnot include glass material in the CCL 42, and the circular points arefor exemplary interconnects that included glass material in the CCL. Ascan be seen in FIG. 10, the exemplary interconnects each had a loweraverage ASRD value (e.g. at least 30% lower, such as at least 50% lower,e.g., 50 to 200% lower) than that of the comparative interconnects after4000 hours of testing.

According to various embodiments, it was determined that is the additionof a glass sintering aid improve the performance of CCLs by reducingarea specific resistance degradation. In addition, the addition of glassalso allowed for CCLs to be self-healing, with the ability to healcracks formed during thermal cycling.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the invention is not so limited. It will occurto those of ordinary skill in the art that various modifications may bemade to the disclosed embodiments and that such modifications areintended to be within the scope of the invention. All of thepublications, patent applications and patents cited herein areincorporated herein by reference in their entirety.

What is claimed is:
 1. A method of forming an interconnect for a solidoxide fuel cell stack, the method comprising: contacting an interconnectpowder located in a die cavity with iron, the interconnect powdercomprising chromium and iron; compressing the interconnect powder toform an interconnect comprising ribs and fuel channels on a first sideof the interconnect, such that the iron is disposed on tips of the ribs;and sintering the interconnect, such that the iron forms an contactlayer on the tips of the ribs having a higher iron concentration than aremainder of the interconnect.
 2. The method of claim 1, wherein thecontacting comprises: filling the die cavity with the interconnectpowder; and disposing the iron on portions of the interconnect powder.3. The method of claim 2, wherein the iron comprises iron wire having athickness ranging from about 20 μm to about 200 μm.
 4. The method ofclaim 3, wherein the disposing comprises disposing the iron wire on theinterconnect powder before compressing the interconnect powder.
 5. Themethod of claim 2, wherein the disposing comprises disposing the ironwire in recesses formed between ribs of an upper punch, and thenbringing the upper punch into contact with the interconnect powder. 6.The method of claim 2, wherein the iron comprises an iron powder havingan average particle size ranging from about 30 μm to about 300 μm. 7.The method of claim 6, wherein the contacting comprises disposing theiron powder in the die cavity, between recesses formed between ribs of alower punch, and then filling the die cavity with the interconnectpowder.
 8. The method of claim 6, wherein the contacting comprisesdisposing the iron powder in recesses formed between ribs of an upperpunch, and then bringing the upper punch into contact with theinterconnect powder.
 9. The method of claim 1, wherein the contact layerextends from the tips of the ribs to a depth ranging from about 5 μm toabout 1000 μm.
 10. The method of claim 6, further comprising forming aprotective coating on a second side of the interconnect, the second sidecomprising ribs and air channels.
 11. The method of claim 6, furthercomprising using a first shoe to fill the die cavity with theinterconnect powder, wherein the contacting comprises depositing theiron powder on the interconnect powder using a second shoe, by spraying,or by screen printing.
 12. The method of claim 1, wherein the iron formsbumps on the tips of the ribs.
 13. The method of claim 1, wherein thesintering comprises partially co-diffusing the iron and the interconnectpowder.
 14. The method of claim 1, further comprising: forming aprotective coating on a second side of the interconnect; and forming acathode contact layer on the protective coating, the cathode contactlayer comprising an electrically conductive metal oxide material andfrom about 4 wt % to about 10 wt % of a glass material, based on thetotal weight of the cathode contact layer.
 15. A method of forming aninterconnect for a solid oxide fuel cell stack, the method comprising:filling a die cavity with an interconnect powder comprising a chromiumalloy; compressing the interconnect powder to form an interconnectcomprising ribs configured to form fuel channels on a first side of theinterconnect; disposing iron on tips of the ribs; and sintering theinterconnect, such that the iron forms a contact layer on the tips ofthe ribs.
 16. The method of claim 15, wherein the iron comprises ironwires having a thickness ranging from about 20 μm to about 200 μm. 17.The method of claim 15, wherein the iron comprises an iron powder havingan average particle size ranging from about 30 μm to about 300 μm. 18.The method of claim 17, wherein the iron powder is disposed on the ribtips by spraying or by screen printing.
 19. The method of claim 17,further comprising: forming a protective coating on a second side of theinterconnect; and forming a cathode contact layer on the protectivecoating, the cathode contact layer comprising an electrically conductivemetal oxide material and from about 4 wt % to about 10 wt % of a glassmaterial, based on the total weight of the cathode contact layer.
 20. Afuel cell interconnect, comprising: oxidant channels at least partiallydefined by first ribs disposed on a first side of the interconnect; fuelchannels at least partially defined by second ribs disposed on anopposing second side of the interconnect; and a cathode contact layerlocated only over tips of the second ribs, the cathode contact layercomprising a conductive metal oxide and a glass material having a glasstransition temperature of 900° C. or less.
 21. The interconnect of claim20, further comprising a protective layer disposed on tips of the secondribs, wherein the cathode contact layer is disposed on the protectivelayer over tips of the second ribs but not over sidewalls of the secondribs or bottom of the oxidant channels.
 22. The interconnect of claim20, wherein: the glass material has a glass transition temperature ofranging from about 450° C. to about 550° C.; and the cathode contactlayer comprises from about 4 wt % to about 10 wt % of the glassmaterial, based on the total weight of the cathode contact layer. 23.The interconnect of claim 20, wherein the conductive metal oxidecomprises La_(1−x)Sr_(x)MnO₃, where 0.1≤x≤0.3.
 24. The interconnect ofclaim 20, further comprising a contact layer disposed on tips of thefirst ribs, the contact layer comprising from about 20 wt % to about 80wt % iron, based on the total weight of the contact layer.
 25. A solidfuel cell stack comprising: solid oxide fuel cells; and interconnects ofclaim 21 disposed between the solid oxide fuel cells, wherein thecathode contact layers contact cathodes of the solid oxide fuel cells.